Carbon-carbon composite clutch friction materials for clutch plates operating in a liquid lubricant (sometimes called "wet") environment have gained acceptance over the past 20 years. Woven fabrics of carbonized or graphitized polyacrylonitrile (PAN) or rayon fibers have been used as a substrate for infiltration with more carbon. The woven carbon fabric, often a plain or square weave pattern, provides a flexible, open mesh or open porosity precursor for a carefully controlled amount of infiltration with carbon to produce a composite product still having suitable porosity and flexibility for clutch operation in a liquid lubricant. The carbon infiltrated, carbon fabric is then glued to a steel disk or plate, often in the shape of a ring, to complete the clutch plate.
In the manufacture of the composite, one or two layers of the woven carbon fiber fabric are subjected to a carbon infiltration process. Sometimes the fabric is infiltrated with a carbonizable thermosetting resin and heated in a furnace under a suitable atmosphere to carbonize the resin. Generally, the fabric is placed in a furnace and the furnace is evacuated of air and heated to 1900.degree. F. or so. A carbonaceous gas, typically natural gas or methane, is flowed around the fabric. The gas decomposes to deposit carbon (pyrolytic carbon) in the fabric. This is the chemical vapor deposition (CVD) process. The carbon infiltration process is continued or repeated until a composite of specified density and porosity is obtained. The friction surface of the composite is usually machined with a surface grinder to remove high spots from the infiltrated fabric.
Heretofore, composites for clutch applications have had a density of about 0.7 to 1.2 grams per cubic centimeter (g/cc). This density range has provided suitable open porosity for infiltration by the wet lubricant and for adhesive infiltration at one side for bonding the composite fabric to a metal plate or disk. For example, Bauer, U.S. Pat. No. 4,291,794, describes clutch materials exhibiting open porosity of 15% to 85% in composite sheets of about 0.030 to 0.033 inch thickness and weight per unit area of 350 to 800 gms/square meter. Thus, the composite sheets had a density of about 0.4 to 1.0 g/cc.
Winckler, U.S. Pat. No. 4,700,823, describes carbon composite clutch friction material produced in the same way. The composite product had open porosity with the original texture of the precursor carbon fabric still being present following carbon deposition by CVD. The patent specification does not disclose density figures, but the Examiner was told during prosecution of the application that materials having bulk densities of 0.7 to 1.2 g/cc provided the advantages of the Winckler invention while providing the required open porosity. A later patent to Collins and Winckler, U.S. Pat. No. 4,828,089, affirmed a preferred bulk density of 0.7 to 1.2 g/cc for this wet lubricated pyrolytic carbon composite friction material.
A group of patents to Genise, U.S. Pat. Nos. 4,844,218, 5,033,596 and 5,221,401, describe applications for substantially the same porous CVD carbon-carbon fiber composites as suitably having a density range of 0.3 to 1.3 g/cc with an optimum range of 0.7 to 1.1 g/cc. Thus, the pyrolytic carbon infiltrated carbon fiber composites for wet clutch applications have all been specified to require the flexibility and open porosity afforded by densities below 1.3 g/cc and preferably below 1.1 g/cc.
Clutch applications have now been encountered in which the above-described low density composite materials have not worked. For example, a torque converter design has involved especially high unit loadings on the clutch and high rate continuous slip operation. The above, now standard, friction materials have not displayed the durability or friction level stability required for such demanding applications. A new, more robust material is required if the use of expensive sintered powder metal or flame sprayed friction materials is to be avoided.